1. Field of the Invention
The present invention relates to a deterioration detecting apparatus for a catalyst which is provided in an exhaust system of an internal combustion engine for purifying exhaust gases and, particularly, to the deterioration detecting apparatus that detects a degree of deterioration of the catalyst.
2. Description of the Related Art
A technique for determining the deterioration of the catalyst provided in an exhaust system of an internal combustion engine for purifying exhaust gases (hereinafter referred to simply as “catalyst”) is conventionally known. For example, Japanese Patent Laid-open No. H02-207159 discloses an apparatus for detecting deterioration of the catalyst. This apparatus measures a first time period CB and/or a second time period CA and determines deterioration of the catalyst based on the first time period CB and/or the second time period CA. The first time period CB is a time period from the time the air-fuel ratio of the air-fuel mixture supplied to the engine is switched from lean to rich with respect to the stoichiometric air-fuel ratio to the time an output of an air-fuel ratio sensor (oxygen concentration sensor) disposed downstream of the catalyst changes to a value indicative of a rich air-fuel ratio. The second time period CA is a time period from the time the air-fuel ratio is switched from rich to lean with respect to the stoichiometric ratio to the time the output of the air-fuel ratio sensor disposed downstream of the catalyst changes to a value indicative of the lean air-fuel ratio.
In the above-described conventional apparatus, the degree of deterioration of the catalyst is detected by the change of the output of the air-fuel ratio sensor disposed downstream of the catalyst by an amount greater than a predetermined value. Accordingly, even when the determination is performed with respect to a normal (not deteriorated) catalyst, it is necessary to change the air-fuel ratio so that the output of the downstream side air-fuel ratio sensor changes by an amount greater than the predetermined value. Therefore, there is a problem that the time period in which the exhaust characteristics are getting worse is likely to be prolonged by performing the deterioration detection.